Process of manufacturing wood pulp in multiple stages



- Patented Feb. 7, 1933 UNITED STATES H NRY x. BENSON, or

PATENT OFFICE SEATTLE, WASHINGTON PROCESS OF MANUFACTURING WOOD PULP IN MULTIPLE STAGES 30 Drawing.

added, heated under pressure, with occasional, accomplished by release of air and gases. This results in loss of heat and in some cases of the reagent. After impregnation the cooking liquor gradually dissolves the encrusting substances from the fibers and is progressively diluted in its active constituents, and finally is discharged as waste together with suspended Z0 fibers. All of the heat contained at the end of the cook is lost together with some of the pulp. Furthermore, in the alkaline process, too high a content of caustic soda, and in the acid process, the development of free sulfuric acidity both produce degraded and deteriorated pulp by reason of contact with spent liquor containing these reagents. In the present art of making pulp, it is obvious, therefore, that heat and material losses occur both at the beginning and at the end of the process and the removal of fiber encrusting substance proceeds slowly by diffusion of the cooking liquor to new surfaces thus exposed. To overcome these losses, I have found it advantageous to reverse the ordinary application of the liquor by adding fresh, cold,

cooking liquor to the hot, nearly cooked batch of pulp. By the transfer of heat from the pulp to this liquor, final purification and refining of the pulp occurs at a moderate temperature with minimum degradation and with good bleachability. I have found that if mechanical agitation and abrasion are empioyed during the main cooking stage, the digestion can materially be accelerated and the plurality of digestors employed in continuous and cascade systems of digesting can be reduced to three, thus eii'ecting a number of economies. The simplification of installation, the saving of heat in two stages, the

Application filed June 22,

1931., Serial No. 546,166.

shortening of the time of cooking in one stage and the removal of pulp in another stage from detrimental impurities of the spent liquor, constitutethe improvement over the present act which my process accomplishes.

I have found that exhausting the air from a sealed digester charged with chips and then admitting hot, nearly spent liquor, produce active boiling under diminished pressure and that the air contained in the chips is replaced m by spent liquor. Two important results are this practice: 1) complete impregnation and (2) heat transfer from spent liquor to chips. In the next stage after I the chips have been freed from excess spent liquor, the main cooking occurs under mechanical agitation with simultaneous abrasion by means of rods, balls or other appliances. The main results accomplished in this stage is the shortening of time of cooking bego cause of removal of encrusting substances and of constant exposure of new surfaces to the chemical reagent. In the final stage, after the separation of the pulp from the cooking liquor, cold fresh liquor is added to the par- :5 tially cooked hot pulp. Two results are accomplished, (1) heat transfer from pulp .to fresh cooking liquor and (2) refining and purification of the pulp with liquor whose composition is controlled to prevent detrian mental effects of sulfuric acidity, or of causticity, on the newly formed fibers.

From this description it can be seen that the novelty of my process resides in (1) The reversal of treatment-of chips with liquor in as controlled stages starting with nearly spent and ending with fresh liquor; (2) the use of mechanica aids to separate the fibers during cooking stages; (3) the heat transfer by heating cold chips with hot liquor from the to main cook; (4) the heat transfer by heating cold liquor in the final stages with hot pulp from the main cook.

1Vl1ilethe prior art (Ger. Pat. 499,010) discloses other processes Where the raw matcrials and liquor for the decomposition move in opposite directions, my process diiiers in that such reverse flow is utilized for the creation of definite stages each under complete mechanical and chemical control of pulp ice making and in one of which stages mechanical abrasion is employed.

1t is obvious that several alternative modes of operating the above described process are poffiiblc. First, a system of rotating digesrcrs can be used and both the wood and liquors can be separated and transferred at the. )tlli)l0ii()l't of each stage; second, a single revolving digestcr can be used, its contents can be removed at the end of each stage, for treatment as desired for the next stage; and third. the stationary digesters now used in the ind ustry can be operated in series of three or more by providing for the discharge and separation of reacting substances at end of each stage.

. It should be noted that the. process above described permits variations in liquorcoir tent imp ssible in the present art. It is generally iecognized, for example, in the sulfatc process that the action of caustic soda at the. end of the cook is harmful to the libers and to prevent such deterioration, the less drastic sodium sulfide is added to the charge at the beginning. In my process wherein mechanical agitation and abrasion are employed to shorten the time. the nearly cooked pulp can be treated with green liquor (Na- .CO- and Nil- S) and the main cook with white liquor ('Na ,S and NaOH).

The following is a more detailed descrip tion of application of my process, of cooking.

in multiple batch stages. Chips and sulphitc acid are charged into a revolving digestcr of acid resistin". metal construction. The air is next evacuated and hot, nearly spent liquor from a second digester is admitted. The temperature of the hot liquor is substantially 100 C. but is rapidly lowered by the cold chips in the revolving digester to substantially C. Under diminished pressure active boiling occurs and more air is expelled from the chips. Upon further cooling, the liquor penetrates the chips and this treatment is complete in substantially one hour.

The chips are now separated from the ore "P55 spent liquor, which is discarded. Tl): chips are then transferred to a rotary (ligt9tct supplied with acid resisting metal rods, '1 nearly fresh sultite cooking liquor is added. The temperature and pressure are gradmltv raised to substantially 120 C. and 7 5 pounds respectively at the end of the cook. The contents of the digester are nextpassed mar a heat insulated rotary filter and the thicltcn d pulp is discharged into a rotary digcstcr containing fresh cold sulfite acid of the usual composition. After agitation, the pulp is again passed over a rotary filter and thickened after which it. passes over the mill screens and deckers in the usual. way.

it is obvious that in commercial procedure and with other species, the precise conditions above described will not apply. but must he. varied to yield the desired pulp. All such ess variations, however, involving treatment of chips in controlled multiple hatch stages in one of which mechanical abrasion is employed, with progressively spent liquor, fall within the scope of my claims.

I claim:

1. Improvements in processes of manufacturing wood pulp, comprising heating wood lilpS with progressively lesser spent reactive liquor, in multiple batch stages, and mechanically abrading the chips in the intermediate stages.

2. improvements in processes of manufacturing wood pulp, comprising as the first stage, the heating of cold wood chips with hot nearly spent cooking liquor which has been separated from the intermediate stages; as the intermediate stages abrading and heating of chips from the first stage with lesser spent liquor from the tinalstage; and as the final stage the treatment. of hot pulp from the intermediate stages with cold fresh liqnor.

3. Improvements in processes of manufacturing wood pulp comprising the mechanical ahrading of wood chips during one of the multiple hatch stages of cooking, substantially a described. I

4 improvements in processes of manufacttlililg wood pulp comprising thc digestion of wood chips with partially spent reactive liquorin the first stages and ahrading the chips during the cooking of the chips in the intermediate stage with nearly fresh cooking liquor. i

Improvements in processes of manufacturing wood pulp comprising the digestion of wood hips in thc first stages. their mechanical abrasion in the intermediate stages and cooking with nearly fresh reactive liquor and the completion of digestion in the last .tagrs by heating with fresh cookinglitpior.

HENRY K. BENSON. 

